Global warming and the greenhouse effect

Greenhouse gases trap heat on Earth

Earth’s atmosphere works something like a giant glass greenhouse. As the sun’s rays enter our atmosphere, most continue on down to the planet’s surface. When they hit the soil or surface waters, those rays release much of their energy as heat. Some of this heat then radiates back into space.

However, several gases in Earth’s atmosphere — such as carbon dioxide, methane and water vapor —work like a blanket to retain much of this heat. That helps to warm our atmosphere. The gases do this by absorbing the heat and radiating it back to Earth’s surface. Such gases are nicknamed “greenhouse gases” because of their heat-trapping effect. Without this so-called greenhouse effect, Earth would be too cold to support most forms of life.

But you can have too much of a good thing. Carbon dioxide is released when people use fossil fuels, such as coal, oil and natural gas. These fuels come from the ancient remains of plants and animals. Products of these fossil fuels, such as gasoline and diesel fuel, power most of the engines that drive cars, airplanes and ships. Coal and other fossil fuels are also burned to run electricity-generating plants that power factories, homes and schools.

By examining air bubbles in ice cores taken from Antarctica, scientists can go back and calculate what the concentrations of carbon dioxide in the atmosphere have been throughout the last 650,000 years. The amount of carbon dioxide in the atmosphere has been climbing to where today it is 30 percent greater than 650,000 years ago.

That rise in carbon dioxide “is essentially entirely due to the burning of fuels,” Susan Solomon says. An atmospheric chemist at the Massachusetts Institute of Technology, outside Boston, she studies factors that affect Earth’s atmosphere and climate.

People have further increased the levels of greenhouse gases in the air by changing the landscape. Plants take up carbon dioxide to make food in a process called photosynthesis. Once cut down, plants can no longer take in carbon dioxide. One result: This gas can begin building up in the air instead of fueling the growth of plants. So by cutting down trees and forests for farmland and other human uses, more carbon dioxide also enters the atmosphere.

“We’ve always had some greenhouse gases in the atmosphere,” Solomon says. “But because we’ve burned a lot of fossil fuels and deforested parts of the planet, we’ve increased the amount of greenhouse gases, and as a result have changed the temperature of the planet.”